Volcanoes Partly to Blame for Global Warming 'Pause'

Below:

Next story in Science

Cooling caused by volcanic eruptions accounts for 15 percent of
the recent global warming "pause," the mismatch between actual
warming and climate-model predictions, according to a new study.

The slowdown in global
warming, sometimes called a pause or hiatus, started in 1998,
when Earth's average surface temperatures halted their feverish
rise. The average rate of warming was 0.31 degrees Fahrenheit
(0.17 degrees Celsius) per decade between 1970 and 1998, but
dropped to 0.072 F (0.04 C) per decade between 1998 and 2012. The
Intergovernmental Panel on Climate Change (IPCC) had predicted
the temperature trends seen in the 20th century to continue at
their disco-era pace.

It turns out that a series of 17 small volcanic
eruptions since 2000 pumped enough aerosols into the
atmosphere to explain a significant portion of the slowdown,
researchers report today (Feb. 23) in the journal Nature
Geoscience. Aerosols are fine, airborne particles — such as
sulfate — that scatter the sun's energy, cooling the Earth. This
cooling has offset the ongoing warming caused by greenhouse gases
such as carbon dioxide, the researchers said. (All told, humans
have released about 100 times more carbon dioxide than the amount
of CO2 belched by volcanoes since 1750, according to the IPCC.)

"Part of the lack of the increase in warming for the last 15
years may be due to the cooling effect of volcanoes," said Céline
Bonfils, a study co-author and climate scientist at Lawrence
Livermore National Laboratory (LNNL) in Livermore, Calif.

"The most recent [climate] simulations include all the major
volcanoes up until Pinatubo in 1991; then the aerosols decay back
to zero," said Mark Zelinka, a study co-author and LLNL climate
scientist. "It was only recently that it was known that these
medium-sized volcanoes were putting a lot of highly reflective
particles into the stratosphere." The stratosphere is the layer
of the atmosphere above the one in which people live (the
troposphere), and extends about 6 to 31 miles (10 to 50
kilometers) above Earth's surface.

In the new study, the researchers correlated 17 volcanic
eruptions since 2000 with shifts in troposphere temperatures, for
which there is a global satellite record of temperature trends.
The same stumble in warming trends since 1998 also hit the
troposphere.

Using computer models and statistical tests, the researchers
calculated that
aerosols from the volcanoes reduced global troposphere
temperatures. The aerosols also cooled the troposphere by
reflecting sunlight.

"We see a statistically significant correlation with not only
temperature, but reflected sunlight — which are both independent
measures," Zelinka said. "That is a pretty key advance."

The results show that the
slowdown in global warming can't be pinned on a single
culprit, the researchers said. Other factors blamed for the
global-warming slowdown include an uptick in sulfur-dioxide
pollution from China and an unusually long minimum in solar
activity. Recent measurements of deep ocean temperatures also
indicate some of the missing heat is being absorbed at deeper
levels in the ocean — a result supported by continued sea level
rise. (Water expands as it warms.)

"The devil is really in the details," said Ryan Neely, an
atmospheric scientist at the National Center for Atmospheric
Research in Boulder, Colo., who was not involved in the study.
"What’s important these days is how do you get down to
decadal-scale predictability in climate change and global
warming, and you have to pay attention to every detail, every
eruption.

"This is the first really rigorous test of whether changes in
volcanic activity relate to tropospheric temperatures, and
they've done a really excellent job," Neely said.